This invention relates to a gas insulated connector capable of interrupting a loop current.
Disconnectors in power station premises are usually installed adjacent to circuit breakers. They are used typically for isolating a circuit after interruption of current therein by a circuit breaker or for switching power transmission systems. The former function is the isolation of a no-load circuit by the disconnector. At the time of the isolation of the no-load circuit by the disconnector, re-arcing repeatedly takes place between the disconnecting switch contacts giving rise to a commonly termed switching surge having high sharpness. This occurs because of the fact that the switching speed of the disconnector is slow compared to the circuit breaker. It has been the practice to add a resistor to the disconnector in order to suppress such a switching surge.
An example of the latter function of the disconnector is the switching of main bus bars in a power station by disconnectors. FIG. 1 shows the case of switching the connection of bus bars A and B in (a) to that in (b) by making use of a satisfactory interrupting capacity of SF.sub.6 gas. In this operation, a current which is close to the rated current in the circuit including the disconnectors A1 and B1 (which is referred to as loop current) is interrupted.
The function of isolating a no-load circuit and the function of switching a loop current, as noted above, are often required for one disconnector in a power station.
A prior art disconnector provided with a resistor for suppressing the switching surge as described above is shown in FIG. 2. The disconnector comprises a tank 1, which is filled with gas capable of extinguishing arc. The tank 1 is sealed by insulating spacers 2 and 3. Conductors 4 and 5 are secured to the respective insulating spacers 2 and 3. A shield member 6 made of a conductor is secured to the conductor 4. The shield member 6a has an opening. A main contact 7 is disposed as a fixed contact in the shield member 6 such that it faces the opening 6a thereof. The main contact 7 is secured to and electrically connected to the shield member 6. A resistor 8 is secured at one end to the shield body 6. The resistor 8 includes an insulating rod 8a and a resisting element 8b as shown in FIG. 6 and as will be described later in detail. A cylindrical spring case 9 is secured to the other end of the resistor 8. A conductive support rod 10 is slidably supported in the spring case 9 and extends in the fixed contact 7. The support rod 10 is provided at one end with a fixed arcing contact 10a which is capable of withstanding arcing. It can be moved up to a position, at which the end of the arcing contact 10a projects from the shield member 6. The arcing contact 10a is connected to the other end of the resistor 8 through the spring case 9. A spring 11 biases the support rod 10 in such a direction that it projects from the shield member 6. Another shield member 12 made of a conductor is secured to the conductor 5. The shield member 12 has an opening 12a. A movable main contact 13 is movably supported in the shield member 12. The main contact 13 is provided at one end with a movable arcing contact 13a which is capable of withstanding arcing. As the main contact 13 is moved to the right from the state of FIG. 2, it is broken apart from the fixed main contact 7 before the arcing contacts 10a and 13a are broken apart. A contact 14 which can be in sliding contact with the movable main contact 13 is connected to the shield member 12. An insulating operating rod 15 is coupled to the main contact 13. A link mechanism 16 transmits a driving force of a driving source (not shown) to the main contact 13 via the insulating operationg rod 15.
In operation, with the rightward movement of the insulating operating rod 15 in FIG. 2, the movable main contact 13 is broken apart from the fixed main contact 7. The arcing contact 10a is caused to follow the movable arcing contact 13a by the action of the spring 11. The arcing contact 10a can be moved up to a position, at which the electric field intensity at the end of the arcing contact 10a is higher than the electric field intensity at all parts of the shield member 6, i.e., at a position at which the end of the arcing contact 10a projects from the shield member 6. The main contact 13 can be moved beyond this position so that arcing is eventually produced between the arcing contacts 10a and 13a. The arcing contact 10a is stopped at a position, at which the electric field intensity at its end is higher than the electric field intensity at all parts of the shield member 6. Thus, a number of arc discharges (re-arcing) all occur between the arcing contacts 10a and 13a during the switching operation. In consequence, an abnormal voltage (i.e., switching surge) generated at the time of isolating the no-load circuit can be suppressed.
FIGS. 3 and 4 are equivalent circuit diagrams showing a succession of states that occur when a loop current is interrupted using a disconnector provided with the surge suppression resistor shown in FIG. 2. In the state of FIG. 3, the disconnector is perfectly closed, and in the stage of FIG. 4 only the main contacts have been broken apart. In FIGS. 3 and 4, the same reference numerals and symbols as in FIG. 2 designate corresponding parts. Indicated at Zm is the impedance of the main current path of the disconnector, indicated at Zl is the impedance of the loop established at the time of the switching of the system, and indicated at R is the resistance of the surge suppression resistor. These impedance values are generally related as EQU Zm&lt;&lt;Zl&lt;&lt;R (1)
The equivalent circuit of FIG. 4 shows the state that results immediately after the start of operation of the disconnector with the surge suppression resistor shown in FIG. 2 to interrupt a loop current. At this time, R and Zm are related as Zm&lt;&lt;R from the inequality 1. If the resistance R is set to be considerably greater than the resistance offered to the arc produced between the main contacts 7 and 13, the loop current scarely flows through the path including the resistor 8, fixed arcing contact 10a and movable arcing contact 13a, that is, it substantially flows through the path including the main contacts 7 and 13. That is, most of the loop current i, for instance about 10 killoamperes, must be interrupted between the main contacts 7 and 13.
In a disconnector capable of interrupting a large loop current, the arcing contacts 10a and 13a usually are capable of withstanding arcing so that an arc due to the loop current can be interrupted between the arcing contacts 10a and 13a for ensuring a current-carrying performance after the interruption of the current. In other words, the main contacts 7 and 13 usually are incapable of withstanding arcing.
If the resistance R of the resistor 8 is set to be comparatively low with respect to the resistance offered to the arcing generated between the main contacts 7 and 13 when these contacts 7 and 13 are broken apart, a large current is caused to flow through the path including the resistor 8 and arcing contacts 10a and 13a with arcing generated between the main contacts 7 and 13 when the contacts 7 and 13 are broken apart. A voltage drop across the resistor 8 is thus applied between the main contacts 7 and 13. In this case, the interruption of the loop current is extremely difficult compared to the case of absence of the resistor 8, i.e., the case where the voltage drop across the resistor 8 is zero. If the resistance R of the resistor 8 is set to be very low, even the function of surge suppression substantially cannot be obtained.
For the above reasons, it has been thought difficult to add a function of interrupting loop current to the conventional disconnector with a surge suppression resistor.